Iron(II)- and Manganese(II)-Induced Oxidation of Limonene by Dioxygen

Monoterpenes are widely used in flavor and fragrance industry. Among these compounds R-(+)-limonene (D-limonene) is the most abundant, low-priced, naturally occurring product, which is suitable feedstock for production of important compounds.

We have found that labile iron(II), iron(III), and manganese(II) complexes {[FeII(bpy)22+]solv, [FeIII(bpy)23+]solv , and [MnII(bpy)22+]MeCN } in acetonitrile activate dioxygen for the oxidation of limonene to produce mainly carvone, carveol, limonene oxide, and perillaldehyde.

Iron(III) complex is reduced by the substrate to respective iron(II) one, which activates dioxygen. Probably the catalyst interacts with limonene prior to the oxidation process. Perillaldehyde is likely formed directly from oxidation of methyl group (not via alcohol). However, the aldehyde is also reduced to perillyl alcohol by the reduced form of the catalyst.

In the case of manganese(II) used as a catalyst the reaction efficiencies after 24 h reaction time are approximately 5-times higher than those obtained for analogous iron(II) complexes. However, the 5 h long induction period is observed for the common conditions of the reaction. The simultaneous presence of the catalyst, dioxygen and the substrate is necessary for the active species to be formed. When t-BuOOH is present in the reaction mixture, the induction period does not appear. In contrast, the replacement of t-BuOOH by HOOH completely inhibits the reaction. This is, to our best knowledge, the first case when manganese(II) complex activates dioxygen for oxygenation process. We have proposed a putative mechanism, in which a manganese(IV)-hydroperoxo adduct with incorporated substrate is formed during the induction period and it becomes an active catalyst for limonene oxidation.

Immobilization of bis(2,2’ bipirydine) -manganese(II) and -iron(II) complexes using bentonite as a support has led to the loss of their activity but higher selectivity of carvone was observed. Moreover, heterogenization of the homogenous catalyst eliminated the necessity of using solvent in the process. In addition, complexes included in bentonite can be reused at least for three times without a detectable catalyst leaching and a significant loss of their activity.

References
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A. Szczepanik, A. Sobkowiak, Catal. Lett. 126 (2008) 261.